Radioisotope capsule protection structure

ABSTRACT

For safety reasons and to prolong the life of a radioisotope capsule and to increase the operating temperature, a radioisotope fuel is enclosed by an inner inert member and a high strength second member and an inert outer member. A vent is provided for release of the helium generated by isotope decay to avoid high pressure within the capsule.

[ 1 Mar. 27, 1973 United States Patent 1 Aranguren et al.

....250/l08 ......l76/67 ....250/l06 .250/l06 S 541 RADIOISOTOPE CAPSULE3,132,998 5 1964 Longetal PROTECTION STRUCTURE 3,432,666 3/ I969 Nash etal....... [75] lnventors: Jorge Aranguren, Playa Del Rey, 31 5 2;

Tex.; John C. Stansel, Palos Verdes, 3'488'502 ["970 Dukes Cam RedondoBeach, Calif. Primary Examiner-Archie R.,Borchelt Attorney-Daniel T.Anderson, Donald William B. Leach W. Graves and Appl. No.: 721,140

' ABSTRACT For safety reasons and to prolong the life of a [52] U.S.Cl.

[51] Int. 1/00 .250/106 S, 108; 176/66, 67

radioisotope capsule and to increase the operating [58] Field ofsearch-mm temperature, -a radioisotope fuel is enclosed by an th second[56] member and an inert outer member. A vent is proinner inert memberand a high streng References Cited UNITED STATES PATENTS vided forrelease of the helium generated by isotope decay to avoid high pressurewithin the capsule.

.250/ 106 S 7 Claims, 3 Drawing Figures 2,830,190 4/1958Karp.................... 3,1l9,933 1/1964 PATENTEI] MR2"! I975 sum 2 OF2 orge Aronguren John O. Stonsel ATTORNEY RADIOISOTOPE CAPSULEPROTECTION STRUCTURE BACKGROUND OF THE INVENTION This invention relatesto radioisotope capsules which provide heat for heating a propellant ina rocket engine, thermoelectric power conversion devices and othersystems where a heat source is necessary.

Radioisotope capsules used in rocket engines have been known in the art.An example is seen in the US. Patent to Martinez et al U.S. Pat. No.3,315,471 and assigned to the assignee of this invention.

One limitation of the typical radioisotope powered thruster is themaximum temperatures to which the encapsulating materials can besubjected due to the pressure build up resulting from the generation ofhelium gas due to the alpha emission of the radioisotope during nucleardecay and structural limitations of materials at high temperatures. Aninterim solution to this problem is provided in the U. S. Patentapplication to Simms, Ser. No. 569,282 assigned to the assignee of thisinvention in which the temperature of the radioisotope capsule ismaintained at a predetermined temperature while the efficiency of theexpanding gas is increased by the addition of an electric heaterdownstream of the capsule. 7

Another problem involved in previous capsule designs resides in thenecessity for establishing at least a minimum safety feature. Forexample, there may be occasions where the capsule over heats, resultingin weakened materials and high pressures sufficient to blow up thecapsule. This could occur during re-entry when the capsule is exposed tohigh external heating rates. Finally, for safety reasons, it isnecessary to provide impact resistance so that a capsule impacting theearth will not fail and release the. radioisotope fuel which wouldobviously create a hazard.

SUMMARY OF THE INVENTION Briefly, the invention is directed to an innercore of radioisotope fuel which is enclosed or surrounded by a firstliner which is substantially inert relative to the fuel and possibleexternal environments. This liner is surrounded by a second linerconstructed from a material such as a refractory metal which providessufficient protection to assure inner liner integrity in the event ofcapsule impact. This refractory metal, however, is subject to corrosionand oxidation. Accordingly, an outer inert liner or clad is provided toenclose the refractory member. Finally, a vent is provided which allowspassage of helium to prevent internal pressure build up but which isimpervious to the passage of radioactive fuel particles. For example, aporous ceramic material can be utilized in the vent.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a view in cross section ofone embodiment of this invention,

FIG. 2 is a view in cross section of another embodiment of thisinvention, and

FIG. 3 is a perspective view, partially cut away, of still anotherembodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a core2'of radioisotope fuel is shown. As an example, this core would compriseplutonium dioxide which structurally consists of small diameter spheresin the range of 50-250 microns in diameter. This core, in turn, issurrounded by a first liner 4 which is one of the noble metals such asplatinum, rhodium, and iridium and any of their intraalloys. This metalis inert relative to the plutonium dioxide and to oxidation andcorrosion. To provide structural rigidity and sufficient protection toassure inner liner integrity in the event of capsule impact, a secondliner 6 is provided which in the preferred embodiment is a refractorymetal such as tantalum, molybdenum, columbium, tungsten, rhenium andtheir alloys. Since this second liner is subject to corrosion by, forexample, oxygen, an outer member 8 surrounds liner 6 to prevent such anoccurrence during normal handling and operation. This outer liner 8 islikewise a noble metal or a noble metal alloy.

To prevent build up of helium pressure generated by the alpha emittingfuel, a vent 10 and/or 12 is provided to allow the escape of helium gasfrom core 2 to the externalatmosphere. This vent can be constructed froma porous ceramic such as the oxides of aluminum, zirconium, hafniumand/or yttrium. These materials will allow the passage of helium gas butretain the small diameter spheres or particles of plutonia. Even in theevent the outer liner is breached on impact and the refractory member 6corrodes, the vented inner liner will provide for long term isotopecontainment required to preventhazards.

In FIG. 2, the construction is substantially similar to that of FIG. 1with the exception that the helium vent (or vents) does not reach theexternal environment but is (or are) connected to a space 14 betweeninner members 16 and the second liner 18. In this design, ,the secondliner 18 serves as both an impact protection member and a pressurevessel to contain the helium during normal operation. This configurationis advantageous since it essentially eliminates the possibility ofdamage to the venting device during impact, but requires the capsule tobe designed as a high temperature pressure vessel during normaloperation. In the event of abort, the capsule outer clad 20 might bedamaged and the refractory member 18 corroded away. The vent would thenrelease the contained helium and the inner liner would provide thelong-term isotope containment required to prevent hazards.

Referring to FIG. 3, a third embodiment is illustrated, which. as inprevious cases, includes a radioisotope core 24, a noble metal liner 26,a refractory liner 28, and an outer noble metal member 30. The vents areshown at 32 and 34. The essential difference between this embodiment andthe first embodiment shown in FIG. 1 is through the addition of heatconducting fins 36. By placing these tins, which also may be of a noblemetal, in the radioisotope core, heat is conducted readily to theoutside surface of the capsule. Although not shown, this capsule wouldnormally be placed in a rocket engine such as shown in theaforementioned U. S. Pat. No. 3,315,471 with gas passed around thecapsule whereby to be heated, expanded, passed through a nozzle and thusprovide thrust.

Having described this invention, it is to be understood that it is to belimited only by the scope of the claims appended hereto.

What is claimed is:

1. A radioisotope heat source capsule comprising:

an inner core containing a radioisotope fuel,

a first noble metal liner enclosing said core, said first liner beingsubstantially inert relative to said fuel and external corrosiveenvironments,

a second refractory metal liner enclosing said first liner andconstructed to withstand substantial impact,

a third noble metal liner surrounding said second liner and adapted toprevent corrosion of said second liner during normal operations, and

a vent extending from said inner core to at least the exterior of saidfirst liner,

said vent being adapted to allow passage of helium generated by theradioisotope fuel decay while preventing passage of said fuel.

2. A radioisotope heat source capsule according to claim 1 wherein saidvent extends through said second and third liners to the exterior ofsaid capsule.

3. A radioisotope heat source capsule according to claim 1 wherein saidvent is a porous ceramic material.

4. A radioisotope heat source capsule according to claim 3 wherein saidporous ceramic material is selected from the group consisting of ofaluminum oxide, zirconium oxide, hafnium oxide and yttrium oxide.

5. A radioisotope heat source capsule according to claim 1 wherein saidfirst and third liners comprise a noble metal selected from the groupconsisting of platinum, rhodium, iridium and their alloys.

6. A radioisotope heat source capsule according to claim 1 wherein saidsecond liner is a refractory metal selected from the group consisting oftantalum, molybdenum, columbium, tungsten, rhenium and their alloys.

7. A radioisotope heat source capsule according to claim 1 wherein heatconductive fins are provided in said inner core so as to conduct heatfrom said core to said liners.

1. A radioisotope heat source capsule comprising: an inner corecontaining a radioisotope fuel, a first noble metal liner enclosing saidcore, said first liner being substantially inert relative to said fueland external corrosive environments, a second refractory metal linerenclosing said first liner and constructed to withstand substantialimpact, a third noble metal liner surrounding said second liner andadapted to prevent corrosion of said second liner during normaloperations, and a vEnt extending from said inner core to at least theexterior of said first liner, said vent being adapted to allow passageof helium generated by the radioisotope fuel decay while preventingpassage of said fuel.
 2. A radioisotope heat source capsule according toclaim 1 wherein said vent extends through said second and third linersto the exterior of said capsule.
 3. A radioisotope heat source capsuleaccording to claim 1 wherein said vent is a porous ceramic material. 4.A radioisotope heat source capsule according to claim 3 wherein saidporous ceramic material is selected from the group consisting of ofaluminum oxide, zirconium oxide, hafnium oxide and yttrium oxide.
 5. Aradioisotope heat source capsule according to claim 1 wherein said firstand third liners comprise a noble metal selected from the groupconsisting of platinum, rhodium, iridium and their alloys.
 6. Aradioisotope heat source capsule according to claim 1 wherein saidsecond liner is a refractory metal selected from the group consisting oftantalum, molybdenum, columbium, tungsten, rhenium and their alloys. 7.A radioisotope heat source capsule according to claim 1 wherein heatconductive fins are provided in said inner core so as to conduct heatfrom said core to said liners.